The invention relates to mobile telecommunication systems, more particularly to mobile telecommunication systems in which mobile terminals communicate with fixed base stations by means of relay stations, and even more particularly to selection of a most suitable one of a number of relay stations for use in communications between a mobile terminal and a base station. Other aspects relate to architectures for mobile telecommunication systems that employ base stations and relay stations.
Cellular communication systems evolving from third-generation (3G) and fourth-generation (4G) systems may provide data rates well exceeding 25 Mb/s, perhaps even exceeding 100 Mb/s. For a receiver in such a system to be able to operate at an acceptable error rate, a minimum signal to noise ratio (SNR) per bit (Eb/N0) is required. What the minimum Eb/N0 is depends on the modulation and coding schemes used and is on the order of 5 dB for coded Quadrature Phase Shift Keying (QPSK) systems, but can be as high as 25 dB for complex modulation schemes like 64-Quadrature Amplitude Modulation (64-QAM). The energy per bit, Eb, is proportional to the received power Prx, and inversely proportional to the data rate R in accordance with:Eb=Prx−R dB Joule
The received power Prx is related to the transmit power Ptx, the antenna gain G, and the propagation loss PL caused by distance and other radio propagation effects in accordance with:Prx=Ptx+G−PL dB Joule
The higher the data rate R, the more the received power Prx has to increase to keep the energy received per bit Eb at an acceptable minimum level to ensure an acceptable bit error rate (BER). Since the output power Ptx of the transmitters is limited (especially in mobile stations due to battery limitations and limited heat-sink capabilities), the increased received power must be achieved by increasing the gain or decreasing the path loss. The gain can be increased using directional antennas. Smart, adaptive antennas are required since the mobile station's position changes with respect to the base station. Alternatively, or in addition, extra base stations are deployed to reduce the distance between the mobile terminal and the nearest base station.
Cell splitting is a known method for decreasing the distance between the base station (BS) and the mobile station (MS). The idea with cell splitting is to have more than one cell site serve a particular geographical area. Each cell site can thereby cover a smaller area, and accordingly use lower power levels which in turn creates the opportunity to reuse frequencies more times within a given system. Until now, geographical area operators have applied cell splitting to increase capacity in their systems. In the next generation cellular systems, cell splitting will be applied to reduce the mobile station-base station distance.
Cell splitting is costly for the system operators. It requires additional base station equipment as well as real estate at which to place the base stations. Therefore, the use of relay stations (RSs) is being considered as a substitute for the additional base station equipment. Instead of a fully functional base station, the relay station merely receives and forwards the signals between the base station and the mobile station. FIG. 1 illustrates an exemplary arrangement comprising a cell 125 having a base station 120, a mobile station 140, and several relay stations 130, . . . , 135. Since the relative positions of the relay stations with respect to the base station are fixed, simple directional antennas can be used between the base station 120 and each of the relay stations 130, . . . , 135. Each of the relay stations 130, . . . , 135 and base station 120 can therefore communicate with very high data rates without the need for excessive transmit power. Because the distance between the mobile station 140 and the nearest one of the relay stations 130, . . . , 135 is limited, high data rate communication can be supported between the mobile station 140 and the relay station 130, . . . , 135 without the use of excessive transmit power. More background on cellular systems with relay stations can be found in R. Pabst et al., “Relay-based deployment concepts for wireless and mobile broadband radio,” IEEE Comm. Magazine, vol. 42, no. 9, pp. 80-89, 2004. By using directional antennas for the (relatively) long path between the base station 120 and each of the relay stations 130, . . . , 135 and using omni-directional antennas for the (relatively) short path between the relay station 130, . . . , 135 and the mobile station 140, capacity and throughput per user is increased as mutual interference between base stations and relay stations is minimized.
The selection of a relay station has been studied in the past. See, for example, V. Sreng et al., “Relayer selection strategies in cellular networks with peer-to-peer relaying,” IEEE VTC Fall '03, Orlando, Fla., October 2003. Selection criteria can be the distance, the propagation path, carrier to noise ratio (C/N) or carrier to interference ratio (C/I), or a combination of any of these. However, in order to carry out the selection process, a network feature has to be implemented including measurement procedures for supporting the selection process.
In a conventional cellular network, the base stations broadcast a control channel (BCCH) or beacon channel. The mobile station can use this BCCH to obtain information necessary to synchronize itself to the network, to register to the system, and to make and accept calls. The received signal strength indicator (RSSI) and the quality of the BCCH can be used as parameters in the base station selection procedure. For proper selection of the relay station, the relay station could broadcast a BCCH as well. However, this would unnecessarily load the system and use scarce spectrum resources wastefully. Although required only for call setup and handover support, which are relatively infrequent events, the BCCH would be transmitted continuously by the relay stations.
In addition, BCCH channel planning (frequency or code allocation) for the relay stations would be an operator's nightmare.
Accordingly, it is desirable to provide an alternative mechanism for suitably selecting one from a number of relay stations to be used as a conduit for communications between mobile stations and base stations.